Preheat oven to 375 degrees.

Beat softened butter in a mixing bowl on medium to high speed for 30 seconds.

Add sugar and nutmeg and beat until combined.

Beat in egg yolks, vanilla, and eggnog.

Beat in as much flour as possible, then stir in remaining flour with a wooden spoon.

Cover the dough and chill for about 1 hour or until easy to handle.

Shape the dough into 1-inch balls.

Roll the dough balls in slightly beaten egg whites.

Then, roll the balls in finely chopped walnuts.

Place the coated balls 1 inch apart on lightly greased cookie sheets.

Press centers of each ball with your thumb.

Bake for 10 to 12 minutes or until edges are lightly browned.

Cool the cookies on wire racks.

To prepare the filling, beat the softened butter until softened.

Add powdered sugar and beat until fluffy.

Beat in enough eggnog to make a filling of spreading consistency.

Spoon 1/2 teaspoon of filling into the center of each cookie.

Sprinkle with ground nutmeg.